1. Field of the Invention
The present invention relates to a displacement measuring instrument and a displacement measuring method of measuring a displacement of an object.
2. Description of Related Art
A laser interferometer capable of setting alignment freely and reducing Abbe errors as compared to a solid scale has been typically known (for instance, see Document 1: JP-A-5-272913).
In the laser interferometer disclosed in Document 1, 1.06 μm YAG laser and its second harmonic are emitted from a light source to be incident on a beam splitter after their polarization states are adjusted by a half-wave plate. A laser beam reflected by the beam splitter is reflected by a reference mirror and a laser beam passed through the beam splitter is reflected by a probe mirror, so that these reflected beams are incident on the beam splitter again to interfere with each other. Then, the interference fringes of the laser beams are split and signals corresponding to the interference fringes are detected, so that geometric lengths are calculated.
When such a laser interferometer as disclosed in Document 1 is used, nonnegligible interpolation errors occur in a small measurement range of few micrometers. Accordingly, it is difficult to measure an object with sub-nanometer accuracy. Thus, a method for shortening a wavelength of a laser beam, a method for amplifying an optical path length by reciprocating a laser beam more frequently between a laser interferometer and a moving reflector to be measured (for instance, Document 2: Takeshi Hatsuzawa, Kouji Toyoda, Yoshihisa Tanimura, Makoto Nara, Syuuji Toyonaga, Shin-ya Hara, Hirotaka Iwasaki, and Kazuhiko Kondou, “Precise Measurements of Micro-linewidths by using a Micro-Interferometer and a Scanning Electron Microscope”, Journal of the Japan Society for Precision Engineering, Vol. 60, No. 11 (November, 1994), pp. 1582-1585), and a method for using a wavelength variable laser (for instance, Document 3: Tuan BANH QUOC, Yuuta HOSHINO, Masashi ISHIGE, Takeshi KOBAYASHI, and Masato AKETAGAWA, “Development of laser interferometer with picometer resolution using frequency tunable laser—The 5th report: Compensation of the displacement measurement due to the fluctuation of the air refractive index—”, collected papers for spring lecture of the Japan Society for Precision Engineering 2008, F02, pp. 441-442) have been used.
According to the method as disclosed in Document 2, a laser beam is reflected by prisms facing each other so that the laser beam is reciprocated five times between the prisms. Thus, the laser beam has resolution that is optically ten times as high as typical laser beam.
According to the method as disclosed in Document 3, a wavelength is varied using a wavelength variable laser during the measurement, so that the measurement can be conducted without interpolation errors.
When the above-described method of shortening the wavelength of laser beam is employed for enhancing resolution and accuracy of the laser interferometer, the resolution can be enhanced and interpolation errors can be reduced. However, stability of a light source and availability and safety of optical components are reduced when light in an ultraviolet region or an X-ray region having a wavelength shorter than that in a visible range is used, and the instrument is enlarged.
When the method of amplifying an optical path length as disclosed in Document 2 is employed, the wavelength of the laser beam is visually shortened, thereby enhancing accuracy. However, an optical system becomes complicated and light quantity is reduced because of the amplified optical path length. Further, maximum moving speed for measurement is decelerated.
According to the method of using a wavelength variable laser as disclosed in Document 3, a laser beam is dispersed in optical components because the wavelength is dynamically varied during the measurement. Further, high stability of the wavelength is hardly obtained.
Thus, it is difficult for a laser interferometer having a simple arrangement to enhance sub-nanometer measurement accuracy in a narrow measurement range of a few micrometers.